1. Field of the Invention
The present invention generally relates to a synchronous machine, and particularly relates to a permanent magnet type synchronous machine which generates electric power or produces rotation torque through relative rotation between a field system constituted by a permanent magnet structure and an armature structure with an armature winding wound thereon.
2. Description of the Related Art
Synchronous machines, that is, synchronous generators or synchronous motors are roughly classified into two types, that is, synchronous machines of the armature rotation type and of the field system rotation type. Most of those synchronous machines using permanent magnets as their field system are of the field system rotation type. In any case, the number of magnetic poles of a field system and the number of salient poles of an armature often provide a ratio of integers such as (n-1):n, where n is an integer. Further, each unit of a plurality of permanent magnets constituting a ring field system is magnetized in the order of a north pole (hereinafter referred to as "N pole") and a south pole (hereinafter referred to as "S pole") or an S pole and an N pole in the radial direction from the inside toward the outside, so that N and S poles are arranged alternately as a whole in the circumferential direction of this ring field system viewed from the inside to the outside in the radial direction. For example, a three-phase permanent magnet electric rotation machine of such a type is disclosed in Japanese Patent Unexamined Publication No. Sho-63-294243 (corresponding to U.S. Pat. No. 4,774,428).
In such a conventional permanent magnet type synchronous machine, each permanent magnet is magnetized in the order of an N pole and an S pole or an S pole and an N pole in the radial direction from the inside toward the outside in the ring permanent magnet structure of the field system. Accordingly, the strongest attraction force is generated between a certain magnetic pole of the permanent magnet and a certain salient pole of the armature when those poles come just in opposition to each other, while when the salient pole of the armature comes into a valley portion between two adjacent magnetic poles, the attraction force suddenly decreases. Thus, strong cogging is caused in rotation of the armature. There has been therefore a problem that, in the case of a generator, not only strong rotation torque is required but also the power generation is small and changes large, and in the case of a motor, the smooth rotation cannot be effected and the output torque is small.
In order to improve such a cogging problem, Japanese Patent Unexamined Publication No. Sho-59-44957 has proposed a technique in which skew is given to magnetic poles constituting a magnetic field.
However, as a result of study of the present inventor, it has been found that the manner how to give skew to magnetic poles constituting a magnetic field is the problem, and the way of giving skew may change the degree of cogging, and has a large influence on the output.